Removal of deposits from platinum surfaces



United States Patent REMOVAL OF DEPOSITS FROM PLATINUM SURFACES James A. McHard and Robert A. Winger, Midland, Mich., assignors to Dow Corning Corporation, Midland, Mich., a corporation of Michigan No Drawing. Original application October 21, 1953, Serial No. 387,552. Divided and this application November 30, 1953, Serial No. 395,262

4 Claims. (Cl. 134-42) The present invention relates to the removal of siliceous deposits from platinum surfaces and the prevention of the formation of such siliceous deposits on platinum surfaces and is a division of our copending application No. 387,552, filed October 21, 1953, now abandoned.

The ever-increasing use of silicon-containing materials in the commercial world has created new and unexpected problems. One of these problems has been the deactivation of catalytic platinum surfaces, such as the platinum filaments employed in certain combustible gas analyzers. Deactivation results when minute quantities of siliconcontaining compounds are present in the atmosphere and are drawn through the combustion chamber of the gas analyzer. The deposit of siliceous material on the platinum filament prevents the filament from carrying out its normal function, namely, catalyzing the combustion of hydrocarbons or other combustible gases which may be present in the samples of air drawn through the analyzer. Thus, the safety device becomes inoperative. It has been found that by increasing the temperature of the filament to over 1,100 C., the contaminants can be driven off, but filament life is materially reduced by heating to this temperature.

Another example of this problem is found in wire coating towers wherein the solvent is stripped from the wire enamel and passes over a hot platinum-black catalyst and is burned, thus providing some of the heat necessary to strip off more of the solvent from the enamel. This use of the stripped solvent as fuel for heating the tower renders the coating process more economical. Thus, it is desirable for all wire coating operations. However, due to poisoning of the platinum catalyst, this process could not heretofore be employed when siliconcontaining enamels, such as silicone-alkyd resins, were employed. This present invention obviates this difficult N b commercially acceptable method for the removal of the siliceous deposits or for the prevention of the formation of such deposits has heretofore been known. The problem has become so acute that some organizations employing combustible gas analyzers as safety devices are presently refusing to use silicon-containing compounds despite the recognized superiority of the products obtained through their use.

It is an object of the present invention to provlde a method whereby siliceous deposits may be removed from -a catalytic platinum surface without substantial deleterious effect to the said surface. A further object is to provide a method whereby the formation of such siliceous deposits may be prevented.

In accordance with the present invention, the platinum surface is contacted with sulfur hexafluoride at a temperature of from 500 C. to 1,100 C., whereby the siliceous deposits will be removed, and further such deposits will be prevented from forming.

Sulfur hexafluoride is exceptionally well suited for use in this invention, and especially for commercial applications of this invention. Sulfur hexafiuoride is relatively inexpensive, non-toxic, and non-corrosive in the quantities required by this invention.

The method of this invention has two aspects. One is the cleansing of a platinum surface which has become contaminated with a siliceous deposit. This is accomplished by contacting the contaminated surface wlth sulfur hexafiuoride in any desired concentration. Obviousl'y the higher the concentration of the sulfur hexa- 2,699,411 Patented Jan. 11, 1955 fluoride at the platinum surface, the faster will be the removal of the deposit.

The other aspect is that of maintaining the surface of the platinum free of siliceous deposit when the platinum surface is in contact with an atmosphere which contains silicon compounds. In order to do this, the sulfur hexafiuoride must be present in the atmosphere surrounding the platinum to the extent of at least one part by weight of the sulfur hexafluoride to one part by weight of the silicon-containing compound in said atmosphere. The precise amount of sulfur hexafluoride employed is, therefore, dependent upon the amount of silicon-containing material present in the particular atmosphere involved and can be determined with ease by trial and error methods. There is no critical upper limit to the amount of sulfur hexafiuoride which can be used.

The temperatures at which the present invention is operative are limited by the temperature at which pyrolysis of the sulfur hexafluoride occurs and the temperature at which permanent deleterious elfects to the catalytic platinum surface will result. It has been found that complete decomposition of the sulfur hexafluoride is not necessary, and sufiicient decomposition will occur at 500 C. Operating at temperatures below 500 C. is impractical because the amounts of sulfur hexafluoride required would be prohibitively expensive. The platinum may be destroyed by operating at temperatures exceeding 1,100 C. The operative temperature range for the present invention is 500 C. to 1,100 C. The preferred temperature range is 800 C. to 1,000 C. because the deleterious effect of high temperature on the platinum is minimized, and the rate of decomposition of the sulfur hexafluoride is great enough to cleanse platinum.

The effectiveness of the process of this invention is independent of the state of subdivision of the platinum. Thus, it is immaterial whether the platinum is in the form of a solid piece such as a filament or is finely divided as in platinum black.

The following example is illustrative only and is not to be construed as limiting the invention, the scope of which is properly delineated in the appended claims. All parts shown in the example are parts by weight.

Example This example is based on results obtained with a continuous, combustible gas analyzer of the type normally employed for the detection of explosive mixtures of combustible gas and air in mines and factories. The gas analyzer makes use of the constant increase in the resistance of a platinum wire with increasing temperature. When a combustible air-gas mixture passes through the instrument, the platinum catalyzes the oxidation and indicates the danger by its increased resistance to the flow of an electric current. The increase in electrical resistance is caused by an increase in temperature of the platinum wire due to oxidation of the gas. Two similar platinum wire filaments are calibrated together to insure consistent accurate performance of the instrument. One filament is sealed in air and acts as a reference cell on one arm of a Wheatstone bridge circuit. The other filament catalyzes the combustion of the gas samples and acts as the other arm of the bridge circuit. A known resistor and a galvanometer form the other two arms. A variable resistor is placed in series with the galvanometer to balance this bridge circuit.

When an air-gas mixture which is combustible passes into the sample analyzing chamber, the increased resistance of the platinum throws the bridge circuit out of balance. The galvanometer deflects the extent of this change in the circuit.

The above-described analyzer was used. in this example in the following manner. The flow of gas into the analyzing chamber of the combustible gas analyzer was controlled by employing a gas line into which commercial grades of oxygen and nitrogen were introduced to simulate air. Also attached to the line were sources of methane, of a trimethyl silyl end-blocked dimethyl polysiloxane fluid having a viscosity of 0. 65 centistokes, and of sulfur hexafiuoride. Thus, it was possible to introduce an explosive mixture of methane and air into the gas analyzer. To this mixture at silicon-containing compound could be added, thus causing the formation of a coating of siliceous material on the platinum filament. The sulfur hexafluoride could be introduced into the line along with the silicon compound, thus preventing the formation of the siliceous coating or removing the coat ing already formed. The supply of any one or any combination of the gases could be cut off by valve arrangements.

The methane-air standard was first introduced into the gas analyzer to establish that it was in operating condition. The methane-air standard was thereafter introduced at regular intervals to check the sensitivity of the instrument. The simulated air mixture (N2 and 02) was then introduced into the line along with the silicone polymer. The platinum filament was found to be coated with a siliceous deposit and inoperative after a short period of exposure to this mixture. Sulfur hexafiuoride was then introduced into the line at a ratio of 2.5 times the weight of the silicone polymer present in the gas mixture. The filament was purged of the siliceous material and remained free of deposit and operated Well within the necessary limits of accuracy even after the sulfur hexafluoride was reduced to a ratio of 1 part by weight of sulfur hexafluoride to 1 part by weight of silicone polymer. However, further reductions 1n the ratio of sulfur hexafiuoride was found to retard but not prevent the formation of a siliceous deposit. No damage to the filament was noted.

That which is claimed is:

1. A method which comprises contacting a platinum surface with sulfur hexafluoride in the ratio of at least 1 part by weight of sulfur hexafluoride to 1 part by weight of silicon compound in the atmosphere in contact with the platinum surface, at a temperature of from 500 C. to 1,100 C., whereby the platinum surface is kept free of siliceous deposits without substantial damage to or alteration of said platinum surface.

2. A method which comprises contacting a platinum surface contaminated by a siliceous material with sulfur hexafluoride, at a temperature of from 500 C. to 1,100 C. whereby the siliceous material is removed from the platinum surface without substantial damage to or alteration of said platinum surface.

3. A method which comprises contacting a platinum surface with sulfur hexafluoride in the ratio of at least 1 part by weight of sulfur hexafiuoride to 1 part by weight of silicon compound in the atmosphere in contact with the platinum surface, at a temperature of from 800 C. to 1,000 O, whereby the platinum surface is kept free of siliceous deposits.

4. A method which comprises contacting a platinum surface contaminated by a siliceous material with sulfur hexafluoride, at a temperature of from 800 C. to 1,000 C., whereby the siliceous material is removed from the platinum surface without substantial damage to or alteration of said platinum surface.

No references cited. 

2. A METHOD WHICH COMPRISES CONTACTING A PLATINUM SURFACE CONTAMINATED BY A SILICEOUS MATERIAL WITH SULFUR HEXAFLUORIDE, AT A TEMPERATURE OF FROM 500*C. TO 1,100* C. WHEREBY THE SILICEOUS MATERIAL IS REMOVED FROM THE PLATINUM SURFACE WITHOUT SUBSTANTIAL DAMAGE TO OR ALTERATION OF SAID PLATINUM SURFACE. 